1. Field of the Invention
The present invention is related to spread spectrum wireless communications and more particularly to code division multiple access (CDMA) wireless digital transmitters and receivers.
2. Background Description
Third generation (3G) mobile wireless technology devices communicate using a protocol known as code division multiple access (CDMA) wherein numerous signals occupy a single transmission channel. The 3G standard known as CDMA2000 promogated by the International Telecommunications Union (ITU) supports mobile data communications at speeds ranging from 144 Kbps to 2 Mbps. These high 3G data rates present designers with several problems. Transmitters for 3G devices include a transmission filter that is designed, primarily to meet emission requirements. For optimum performance it is desirable that the system receivers have a filter with a transfer function response that is matched to that of the transmitter's filter. Any mismatch between the transmitter filter and the receiver filter impacts the receiver effectiveness by reducing receiver signal noise ratio (SNR).
A typical CDMA2000 base station transmitter signal path includes a finite impulse response (FIR) filter followed by a pre-distortion phase equalizer. The pre-distortion phase equalizer leads to a simplified mobile receiver filter which is, by contrast, a fifth order or sixth order elliptic filter. However, since the frequency response of the standard transmitter's FIR filter and predistortion equalizer cannot be matched to the frequency response of the fifth or sixth order elliptic filter, the SNR cannot be maximized for the CDMA2000 systems using current receiver designs.
Also, what is known as Inter-Chip Interference (ICI) occurs when reflected signals are delayed long enough that successive bits (or chips) in the demodulated signals overlap, creating uncertainty in the receiver data. Second generation and older systems with larger signals transmitted at a lower data rate (i.e., for voice and other low data rate applications) could tolerate this lower SNR. Unfortunately, at the higher packet speed 3G data rates and beyond (1–5 Mbps) and smaller signals that use 16 or 64 bit quadrature amplitude modulation (QAM), these same filter mismatches cause an intolerable SNR increasing ICI in the receiver, i.e., on the order of 3 dB, and impact receiver performance. For backward compatibility, the same baseband filter in existing IS95/CDMA2000 systems filter high speed data packets. As a result, the current IS95/CDMA2000 based mobile baseband processor suffers from very high ICI in high speed data packets and significantly degraded system performance.
Thus, there is a need for a wireless receiver with improved signal to noise ratio that is less sensitive to inter-chip interferences.